(Adapted from the applicant's abstract) The aim of the present study is to quantify and empirically evaluate a model of photopic detection and light adaptation. The experiments combine two approaches to the study of photopic sensitivity: the "adaptation" and the "color vision" approaches. The former examines foveal detection and adaptation with the probe-flash paradigm. Increment thresholds are measured for a brief target (the "probe") presented upon a flashed field (the "flash) to which the visual system has not had time to adapt. The probe flash techniques reveals nonlinearities in the cone system that are not evident with steady fields. In the color vision approach, models originally formulated to explain color appearance are applied to sensitivity data. The models assume that the detectability of a light depends on the activity it elicits in three pathways or channels. These include two opponent chromatic channels (red/green and yellow/blue) and one nonopponent achromatic pathway. Probe-flash data have been fit with models containing a single static nonlinearity. While the data are easily described by such models, the site of sensitivity loss within the visual pathway remains unclear. Psychophysicists initially assumed the data reflected nonlinearities within the cone receptors. However, a variety of measures reveal that under many conditions, desensitization occurs beyond the receptors, and involves spectrally opponent mechanisms. A two-site model was developed to account for the evidence of post- receptor nonlinearities in probe-flash data. One aim of current proposal is to formulate a model that predicts the effects of changes in adapting intensity. A secondary goal is to compare the static nonlinearities inferred psychophysically to primate physiology. Recent recordings of primate cone response functions provide a new opportunity for direct comparison between receptor physiology and behavior. Sensitivity in both long- and short-wavelength cone pathways will be examined. The latter provides a valuable subject of study because of its relative vulnerability to retinal disease. Probe-flash data obtained for these patients have generated some disagreement as to whether the observed falloff in sensitivity reflects receptor or post-receptor deficits. By isolating receptor and opponent nonlinearities in normal observers, and comparing the results to primate receptor data, some insights into the locus of sensitivity loss in certain clinical populations can be gained.